826 J. BAHRELL MEASUREMENTS OF GEiOLOGlC TIME 



ill semi-arid to arid regions the oscillations ichange to a large degree the 

 quantity of life which can exist and compel the rapid migration of 

 faunas. Such changes find geologic record through their effects on the 

 carrying power of streams and the depth of wave action. There have 

 been important fluctuations measured in centuries, and still more im- 

 portant ones which icover several thousand years. They appear, however, 

 to be rather siidden and irregular in their inception. According to 

 Huntington, the climate through the primitive historic era, dating from 

 3,000 -f- B. C. to 1,200 B. C. was on the average markedly cold and wet, 

 but still showing oscillations. During the period from 1,000 B. C. to 

 500 A. D. there was a decrease in these conditions, and from 600 A. D. 

 to the present there has existed a period of general warmth and aridity.*'^ 



The work of Huntington joins on to that of the glacial geologists. 

 Periods measured in centuries and in some thousands of years should 

 find notable expression in glacial oscillations and result in the building 

 of retreatal moraines. These climatic oscillations during the historic 

 period are geologically of importance, in that they show the existence of 

 such rhythms which are not due to the precessional period of 21,000 

 years, but which, if found recorded in the strata, might have been mis- 

 taken for il. 



F. B. Taylor has published a valuable study on the moraines of reces- 

 sion which marked the retreat of the last ice-sheet from the Ohio Eiver 

 to the Great Lakes.^^ According to Taylor : 



"1. Between Cincinnati and Mackinac the Wisconsin drift formation has 

 fifteen terminal moraines which form a consecutive series marking the retreat 

 of the last ice-sheet, and there are three more farther north probably belonging 

 to the same set. The series seems to be complete and is believed to constitute 

 the simplest and most perfect known. 



"2. Making due allowance for the influence of topography, it appears that 

 the intervals between the members of the series are remarkably regular, sug- 

 gesting periodic halts or oscillations of the retreating ice-front, which appear 

 to be attributable only to a periodic change of climate." ^^ 



Taylor tries to fit the series to the precessional cycle, as the only known 

 cause, but finds 21,000 years too long. He states that a period of be- 

 tween 5,000 and 10,000 years would seem to accord most closely with 

 the phenomena. 



In Europe the retreat of the last ice-sheet was marked by three greater 

 halts. These, beginning with the maximum, are as follows: 



^•^ E. Huntington : Palestine and its transformations, chap, xvi, 1911. 

 "" Moraines of recession and their significance in glacial theory. .Tour. Geol., vol. 

 1.S07, pp. 421-40r). 

 ™ Loc. cit., p. 464. 



